There is a present trend to operate internal combustion engines at higher temperatures and engine speeds to provide improved fuel efficiency and increased power output. The trend to such higher temperatures and speeds places severe demands and increases wear on the engine's reciprocating components such as valve components and poppet valves in particular.
As the mass of the reciprocating components becomes a hinderance to higher speed operations, there is a growing interest to minimize the weights of the reciprocating components.
Titanium has been found to possess lighter weight and high temperature resistance characteristics attractively advantageous for making engine poppet valves operating at such higher temperature and speeds.
It has been found however, that the wear resistance and, in some instances, the strength and ductility of titanium requires improvement to insure long term engine operation at such higher temperatures and speeds as, for example, where it has been the practice to cap or otherwise provide a wear resistant material at the tip of the valve stem which is a region that has been found to be particularly subject to wear.
A variety of approaches have been taken in the past to improve either or both the strength and wear resistance of titanium valves for use under such higher engine operating temperature and speeds.
One approach to increasing the strength of a titanium valve stem is disclosed in U.S. Pat. No. 4,852,531, the disclosure of which is incorporated herein by reference, where particles of compounds selected from the group consisting of titanium carbide, titanium boride, and titanium diboride are mixed in prescribed amounts with powdered titanium in the making of the valve stem.
Another approach to improving the wear resistance of titanium is disclosed in U.S. Pat. No. 5,051,140, the disclosure of which is incorporated herein by reference. Here, the surface of the titanium article is cleaned with an acid and heated in an oxidative atmosphere to provide a composite layering thereupon of oxide and oxygen-enriched layers.
Another method for improving the performance of titanium engine valves at higher operating temperatures and speeds is disclosed in U.S. Pat. No. 4,675,964, the disclosure of which is incorporated herein by referenced. Here, the valve head is heated and worked to provide a colony type microstructure highly resistant to heat to provide a mixture of five equiaxed alpha and transformed beta crystalline grains exhibiting high resistance to tensile shock and fatigue.
The application of titanium nitride coatings by a reactive arc vapor deposition process for improving erosion resistance of titanium alloy turbine blades is known and described for example in U.S. Pat. No. 4,904,528, the disclosure of which is incorporated herein by reference. This patent as well as U.S. Pat. Nos. 4,929,322 and 5,066,515, the disclosures of which are incorporated herein by reference, describe what is essentially a plasma arc method of applying a suitable coating on a substrate that requires the creation of a vacuum which is not the case for the process of the present invention which involves an electro deposition process utilizing pressurized nitrogen.
The present invention is directed to the discovery of a method for forming a titanium nitride coating on the surface of metal valves in one embodiment and the formation of a zone of titanium nitride in situ extending inwardly from the outer surface of a titanium valve in another embodiment and to a combination of the two in a third embodiment to enhance abrasion resistance at the higher engine temperature and speeds. All three embodiments involve alteration of the metal metallurgical microstructure and exposure to ionized nitrogen at nitriding temperatures while an electrical potential is imposed between the valve and a cathode to provide the coating and/or in situ zone of titanium nitride that is substantially uniform in thickness and strongly adhered to the metal substrate therebeneath.